2 3 6 



PRACTICAL MICROSCOPY. 



eye-piece, the whole being moved up and down until the 

 edge of the disc is seen quite distinctly ; the procedure is 

 then the same as with low powers. This auxiliary objec- 

 tive must be furnished with a diaphragm to cut off all false 

 rays, the aperture being one-eighth of an inch in diameter, 

 and situated i 5 inches from the achromatic combination. 



The numerical aperture divided by the refractive index 

 of the medium in which it is measured produces the sine 

 of the semi-angle, and thus the angle in any medium may 

 be easily obtained. 



The following table of natural sines may be of use in 

 this direction : 



NATURAL SINES. 



